The present invention relates to a coated substrate. More particularly, the invention relates to a substrate coated with a tin oxide-containing material, preferably an electrically conductive tin oxide-containing material.
Even though there has been considerable study of alternative electrochemical systems, the lead-acid battery is still the battery of choice for general purposes, such as starting an automotive vehicle, boat or airplane engine, emergency lighting, electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware, both industrial and military. These batteries may be periodically charged from a generator.
The conventional lead-acid battery is a multi-cell structure. Each cell comprises a set of vertical positive and negative plates formed of lead-acid alloy grids containing layers of electrochemically active pastes. The paste on the positive plate when charged comprises lead dioxide, which is the positive active material, and the negative plate contains a negative active material such as sponge lead. An acid electrolyte, based on sulfuric acid, is interposed between the positive and negative plates.
Lead-acid batteries are inherently heavy due to use of the heavy metal lead in constructing the plates. Modern attempts to produce light-weight lead-acid batteries, especially in the aircraft, electric car and automotive vehicle fields, have placed their emphasis on producing thinner plates from lighter weight materials used in place of and in combination with lead. The thinner plates allow the use of more plates for a given volume, thus increasing the power density.
Higher voltages are provided in a bipolar battery including bipolar plates capable of through-plate conduction to serially connected electrodes or cells. The bipolar plates must be impervious to electrolyte and be electrically conductive to provide a serial connection between electrodes.
U.S. Pat. Nos. 4,275,130; 4,353,969; 4,405,697; 4,539,268; 4,507,372; 4,542,082; and 4,510,219; relate to various aspects of lead-acid batteries. Certain of these patents discuss various aspects of bipolar plates.
Attempts have been made to improve the conductivity and strength of bipolar plates. Such attempts include the use of conductive carbon particles or filaments such as carbon, graphite or metal in a resin binder. However, such carbon-containing materials are oxidized in the aggressive electrochemical environment of the positive plates in the lead-acid cell to acetic acid, which in turn reacts with the lead ion to form lead acetate, which is soluble in sulfuric acid. Thus, the active material is gradually depleted from the paste and ties up the lead as a salt which does not contribute to the production or storage of electricity.
The metals fare no better; most metals are not capable of withstanding the high potential and strong acid environment present at the positive plates of a lead-acid battery. While some metals, such as platinum, are electrochemically stable, their prohibitive cost prevents their use in high volume commercial applications of the lead-acid battery.
In "Preparation of Thick Crystalline Films of Tin Oxide and Porous Glass Partially Filled with Tin Oxide", by R. G. Bartholomew et al, J. Electrochem, Soc. Vol 116, No. 9, p 1205(1969), a method is desribed for producing films of SnO.sub.2 on a 96% silica glass substrate by oxidation of stannous chloride. The plates of glass are pretreated to remove moisture, and the entire coating process appears to have been done under anhydrous conditions. Specific electrical resistivity values for SnO.sub.2 -porous glass were surprisingly high. In addition, doping with SbCl.sub.3 was attempted, but substantially no improvement, i.e., reduction, in electrical resistivity was observed. Apparently, no effective amount of antimony was incorporated. No other dopant materials were disclosed.
In "Physical Properties of Tin Oxide Films Deposited by Oxidation of SnCl.sub.2 ", by N. Srinvasa Murty et al, Thin Solid Filmas, 92(1982) 347-354, a method for depositing SnO.sub.2 films was disclosed which involved contacting a substrate with a combined vapor of SnCl2 and oxygen. Although no dopants were used, dopant elements such as antimony and fluorine were postulated as being useful to reduce the electrical resistivity of the SnO.sub.2 films.